Method of removing impurities in a molten metal bath for glass sheet making



July l1. 1967 D G, LOUKES ET AL 3,330,637

l METHOD OF REMOVING IMPURITIES IN A MOLTEN METAL BATH FOR GLASS SHEET MAKING v Filed Aug. 17, 1964 4 Sheets-Sheet l July 1l. 1967 v G` LouKl-:s ET AL 3,330,637

METHOD OF REMOV IMPURITIES IN A MCLTEN METAL BATH FOR GLA NG l ss SHEET MAKI 4 Sheets-Sheet 2 Filed Aug. 17, 1964 Inventory Mtorney;

July 11, 1967 l D G. LOUKES ET AL 3,330,637 BATH METHOD OF REMOVING IMPURITIES IN A MOLTEN METAL FOR GLASS SHEET MAKING 4 Sheets-Sheet 5 Filed Aug. 17, 1964 O@ O@ Q Mm@ if om pw wm @Q .Jwowwm wm Kw. L o I, :Adv NU mmuwmwmmblolwmw O www@ w u f@ w Q A /7/ 4J- VM/// y w A m n ow mm, `www am www om, mm Q Q July 1l, 1967 D G, OUKES ET AL 3,330,637

METHOD OF' REMOVIG IMPURITIES IN A MOLTEN METAL BATH FOR GLASS SHEET MAKING Filed Aug. 17, 1964 4 Sheets-Sheet 4 United States Patent O Britain Filed Aug. 17, 1964, Ser. No. 390,014 Claims priority, application Great Britain, Aug. 19, 1963, 32,758/63; Feb. 27, 1964, 8,260/64 This invention relates to the manufacture of flat glass and in particular to the manufacture of flat glass during which the glass is in Contact with molten metal, for eX- ample during which the glass is advanced in ribbon form over a bath of molten metal.

The molten metal is, for example, molten tin or a tin alloy having a specific gravity greater than glass, and when employing a bath of molten metal the bath is, for example, so constituted as to have all the characteristics fully described in U.S. Patent Nos. 2,911,759 and 3,083,551.

Impurities, for example oxygen and/or sulphur may be present in the molten metal, and it is a main object of the present invention to provide a method of removing such impurities from the molten metal bath.

It is another object of the invention to control the maintenance of a trace of an additive element in the molten metal.

According to the invention, there is provided in the manufacture `of flat glass during which the glass is in contact with molten metal, the steps of maintaining in the molten metal a trace of an additive element with which an impurity in the molten metal preferentially reacts, and removing from the molten metal a product of the reaction of said additive element with an impurity by maintaining in contact with a zone of the molten metal a molten layer of a compound which takes up said product from the molten metal.

Desirably the invention is characterised by confining said molten layer on the surface of the molten metal and continually presenting a fresh surface of said compound to the molten metal at the interface of the layer and the molten metal.

In a preferred embodiment -of the invention a fresh surface of the molten compound is continually presented by applying a controlled electric current to the molten layer so as to cause electrolytic release of the impurity,

and then removing the released impurity from the molten layer.

Further according to the invention, in the manufacture of flat glass during which the glass is in contact with molten metal, a trace of an additive element Withwhich an impurity in the molten metal preferentially reacts is maintained in the molten metal, and a product of the reaction of said additive metal with an impurity is removed from the molten metal by supporting on the surface of a zone of the molten metal a confined molten layer of a fluoride melt containing at least one compound of said additive element which fluoride melt takes up said product from the molten metal.

As an alternative to the electrolytic release of the impurity the reaction product taken up from the molten metal may be removed by continuously decanting the molten compound from the surface of the zone of molten metal so that a fresh surface of said compound is continually presented at the interface with the molten metal.

The use of a molten layer of a fluoride has been found to be particularly efficacious, as a fluoride may dissolve some at least of the reaction product. From this aspect 3,339,637 Patented July 11, 1967 ICC the invention comprehends, in the manufacture of flat glass during which glass is in contact with molten metal, scavenging an impurity from the molten metal by maintaining in the molten metal a trace of an additive element with which said impurity preferentially reacts, supporting on the surface of a Zone of the molten metal a confined molten layer of a fluoride melt containing at least one compound of the additive element, which fluoride melt takes up from the zone the product of the reaction of the additive element with said impurity, and applying a controlled electric current to the fluoride melt so as to cause electrolytic release of the impurity from the fluoride layer.

Preferably the zone of molten metal acts as a negative electrode for the electrolytic action, so that additive element released by electrolysis of the reaction product taken up by the melt returns to the molten metal to maintain therein the desired trace of additive element. A cyclic process is thus set up and neither the additive element nor the fluoride layer has to be replenished, except during normal routine checking of the operation of the process.

Desirably according to the invention the molten metal is predominantly molten tin, and the additive element is an element selected from the group consisting of lithium, sodium, potassium, magnesium, calcium, strontium, barium, cerium, manganese, iron, zinc, boron, aluminium, silicon, titanium, zirconium, niobium and tantalum. Oxides of these elements may appear in the form of a fine dross and dross reaching the surface of the molten metal in the zone is taken up in the molten layer and the additive element may be subsequently released back into the molten metal by electrolysis when the molten layer is a fluoride melt.

The impurity from the molten metal, for example oxygen or sulphur is also released by the electrolysis and percolates through the molten layer into the head-space over the layer. This headspace over the layer is segregated from the headspace over the bath of molten metal along which the glass is advanced, since the molten layer is usually confined in a recess in the tank structure confining the bath of molten metal, which recess contains a dened pocket of molten metal of the bath; and the pocket is preferably Vsegregated from the headspace over the bath by a partition extending downwardly into the bath metal -to define with the floor of the tank a channel by which the pocket communicates With the bath.

In one method of operation according to the invention the additive element is aluminium, and a confined layer of sodium aluminium fluoride (cryolite), with dissolved alumina, is supported on said zone.

ln another method according to the invention the additive element is boron, and a confined layer of a mixture of potassium fluoborate and potassium chloride, with dissolved boric oxide, is supported on said zone.

In a further method according to the invention the additive element is magnesium, and a confined layer of a mixture of magnesium fluoride, barium fluoride and sodium fluoride, with dissolved magnesium oxide, is supported on said zone.

Further according to the invention the molten layer may be a layer -of a mixed salt selected from the group consisting of lithium potassium chloride, sodium potassium chloride, calcium barium chloride and lithium potassium fluoride.

In order to assist the carrying of reaction products in the molten metal into contact with the molten layer, the layer maybe supported on the surface of the molten metal in the pocket, and a circulation of the molten metal in the pocket is enforced to bring molten metal of the bath carrying reaction products into Contact with said molten layer and to assist the diffusion of the additive element through the bath of molten metal.

The circulation of the molten metal in the pocket may alone provide sufficient assistance to the movement of the reaction product into contact with the molten layer and to assist diffusion of the additive element through the bath of molten metal. However this movement may be assisted by causing a ow of molten metal from the main part of the bath into the pocket, through the pocket and out of the pocket into the main part of the bath.

Further the invention may include heating the pocket of molten metal by inducing upward movement of heated molten metal in the pocket. Such upward movement in the pocket creates a movement of the molten metal at the interface of the molten metal and the supported compound and carries reaction products in the molten metal to the interface, and carries away additive element as soon as it is released at the interface by the electrolytic action. This assists in providing a continuously renewed surface of the molten compound at the interface.

If the temperature of the molten metal is suiciently high the molten layer supported on the molten metal will be maintained in the liquid state by heat transference from the metal. Further according to the invention the supported molten compound may be heated by passing a controlled alternating current through the molten layer. The compound can thus be maintained molten even when the temperature of the molten metal is not suflciently high.

The invention also comprehends apparatus for use in the manufacture of at glass in ribbon form, comprising an elongated tank structure containing a bath of molten metal including a pocket of molten metal formed in the tank structure, means for advancing glass in ribbon form along the bath towards an outlet from the bath, the tank structure being so formed as to contain on the surface of the molten metal in the pocket a molten layer of a compound which takes up from the molten metal a reaction product of an impurity in the molten metal with an element added to the molten metal, and means associated with the pocket for providing continually a fresh surface of said compound at the interface of the compound with the molten metal.

A preferred apparatus according to the invention includes electrical connections so arranged in the pocket as to pass electric current through the molten layer which is supported on the molten metal in the pocket, and means for supplying a controlled electric current to the electrodes to cause electrolysis of the molten layer.

Further according to the invention, in order to maintain a plenum of protective atmosphere over the bath along which the glass in ribbon form is advanced, a roof structure extends over the tank structure to dene a headspace over the bath, and the pocket is located in a side wall of the tank structure and is segregated from said headspace by a partition extending downwardly into the bath metal to deine with the floor of the tank a channel by which the pocket communicates with the bath.

Preferably, one electrode, being for example a carbon electrode, is positioned in the pocket above the surface level of the molten metal, the other electrode is constituted by the molten metal, and the current supply means is so connected to the electrodes that the molten metal in the pocket constitutes a negative electrode.

Still further according to the invention channels may be formed in the tank structure and spaced apart longitudinally of the bath, by which channels the pocket cornmunicates with the main part of the bath, and impeller means in one channel is operable to cause a ow of molten metal through that channel such that molten metal enters the pocket through one channel and returns to the bath through the other channel after circulating through the pocket.

Preferably the impeller means is a paddle wheel mounted in the upstream channel, and a motor connected to the paddle wheel is operable to cause the Wheel to impel molten metal out of the pocket.

The heating of the molten metal in the pocket may be effected by an upward circulation of heated molten metal in the pocket, and in order to induce such an upward movement, an electric induction heater may be mounted in the base of the pocket, communicating with the pocket.

Further according to the invention, in order to heat the supported layer, to maintain it molten, auxiliary heating electrodes are so mounted in the pocket as to dip into the molten layer supported on the molten metal in the pocket, said electrodes being for connection to an A.C. source operable to pass alternating current through the molten compound between the electrodes.

In order that the invention may be more clearly understood some embodiments thereof will now be described, by way of example, with reference to the accompanying drawings in which:

FIGURE 1 is a central longitudinal sectional elevation of apparatus according to the invention comprising an elegonated tank structure containing a bath of molten metal and a superimposed roof structure, and indicating the position of pockets of molten metal in one side wall of the tank structure,

FIGURE 2 is a section on line II-II of FIGURE 1 showing in detail the construction of one of the pockets in the tank side wall,

FIGURE 3 is a view similar to FIGURE 1 embodiment of the invention,

FIGURE 4 is a section on and FIGURE 5 is a section on line V-V of FIGURE 4.

In the drawings the same reference numbers designate the same parts.

Referring to FIG. 1 of the drawings, a forehearth of a continuous glass melting tank is indicated at 1, and a regulating tweel at 2. The forehearth ends in a spout comprising a lip 3 and side jambs 4, one of which is shown. The side jambs 4 and lip 3 form a spout of generally rectangular cross section. A cover may be secured over the spout in known manner.

The spout 3, 4 is disposed above the iloor 5 of a tank structure including side walls 6 joined together by end walls 7 and 8 integral with the side Walls 6 and the oor 5. This tank structure contains a bath 9 of molten metal, for example molten tin or an alloy of tin having a specific gravity greater than glass. The level of the surface .of the bath is indicated at 10.

A roof structure is supported on the tank structure, and the roof structure includes a roof 11, side walls 11a and end walls 12 and 13 respectively at the inlet and outlet ends of the bath. The roof structure thus provides a tunnel over the bath and denes a headspace 14 over the bath.

The lower face 15 of the end wall 12 defines with the surface 10 of the bath an inlet 16, which is restricted in height, for glass 17 in ribbon form as the glass is advanced over the bath. The roof structure is extended to the tweel 2 by a roof element 18, and side walls 19 which form a chamber in which the spout is situated. The lower face of the end wall 13 of the roof structure defines with the end wall 18 of the tank structure an outlet 20 for the ultimate ribbon of glass 21 which is discharged from the bath.

Driven conveying rollers 22 are mounted outside the outlet end of the tank and are disposed somewhat above the level of the top of the tank end wall 8. superimposed driven rollers 23 are provided, and the rollers 22 and 23 co-operate to apply a tractive effort to the ribbon of glass 21 moving towards the outlet 20 from the bath, which tractive effort assists in advancing the ribbon of glass along the bath. The ribbon 21 is directed by the rollers to a conventional tunnel lehr in which the ribbon is annealed, as is well understood in the art, and on leaving the lehr the ribbon is cut into sheets of desired size.

Molten glass is poured on to the bath 9 of molten metal from the spout 3, 4. The tweel 2 regulates the ow of molten glass from the spout lip 4 and the spout is verof another line IV-IV of FIGURE 3,

tically spaced from the Surface of the bath so that the molten glass has a free fall of a few inches to the bath, which distance is such as to ensure a heel 24 of molten glass being formed behind the glass falling from the spout, which heel extends rearwardly to the end Wall 7 of the tank structure.

The temperature of the bath is regulated from the inlet end to the discharge end by providing thermal regulators 25 immersed in the molten metal bath 9. Radiant heaters 26 are provided in the headspace 14 to assist the temperature regulation. The temperature regulators 25 and 26 at the inlet end of the bath are devised to maintain the temperature at the inlet end at about 1,000 C. or slightly higher over a sufficient length of the bath to ensure that the molten glass is advanced along the bath as a buoyant layer of molten glass 17 from which is developed a buoyant body of molten glass 27. The width of the tank structure is somewhat greater than the width 0f the buoyant body 27 at the surface level of the bath so that the buoyant layer 17 is able to flow laterally unhindered to the limit of its free flow to develop the buoyant body 27 of molten glass which is then advanced in ribbon form along the bath.

The temperature regulators 25 and 26 spaced down the bath maintain a temperature gradient such that the ribbon is cooled to a state in which it can be taken off the bath unharmed by mechanical means by the time it nears the outlet end of the bath. That is the ribbon 21 has been progresively cooled to a temperature of about 600 C. before being taken up oil the bath by the conveying rolls 22 as shown in FIG. 1.

The roof structure is at intervals provided with ducting 28 connected by branches 29 to headers 30 through which a protecting gas is fed into the headspace 14 over the bath to create a plenum of protecting gas in the substantially closed headspace. The protecting gas is a gas which will not react chemically with the metal of the bath to produce contaminants for the glass, and therefore protects the bath surface at the sides of the ribbon and under the end of the stiflened ribbon 21 leaving the bath. Ingress of external atmosphere through the restricted inlet 16 and outlet 20 is substantially prevented.

A trace of an additive element is maintained in the molten metal bath 9 to react with impurities in the bath, for example oxygen and/ or sulphur which may be present in the atmosphere over the bath or may migrate into the bath from the glass delivered to the bath. In order to scavenge these impurities from the bath it has been found advantageous to maintain in the bath a trace of an additive element with which the impurities preferentially react rather than with the metal of the bath.

In order to remove impurities from the bath the method of the invention has been devised. Pockets 31 for molten metal of the bath are formed in one or both side walls 6 of the tank structure. Two pockets 31 are indicated in FIGURE l, but there may be more than two pockets spaced at regular intervals down one or both side walls 6. One of the pockets is illustrated in greater detail in FIGURE 2.

As shown in FIGURE 2 the pocket is formed by an extension of the floor 5 of the tank structure with a stepped down formation as shown at 32 in FIGURE 2. The side wall 6 of the tank structure forms a partition extending downwardly into the bath metal 9 towards the floor 5 of the tank structure to define with the floor 5 of the tank a channe-l 34 by which the pocket 31 communicates with the main part of the bath 9. The slot shape of the channel 34 is also indicated in FIGURE 1.

The pocket 31 has an outer wall 35 intergral with the floor 33 and the wall 35 supports a roof 36 extending between the top of the wall 35 and the side wall 11a of the roof structure which rests directly on the side wall 8 of the tank structure. There is thus a headspace 37 over the molten metal in the pocket 31 and a duct 28 connects with this headspace 37 to enable a flow of atmosphere to be maintained through the headspace 37.

A layer 39 of a fluoride of an additive element for the bath is confined in the pocket, being supported on the surface of the molten metal in the pocket. The product of the reaction between the additive element and impurities in the molten metal rises in the form of dross to the surface of the molten metal in the pocket and a layer 39 of a compound which will take up that product from the molten metal is supported on the surface of the molten metal in the pocket. If the layer 39 is a molten layer of a fluoride of an additive element then the reaction product becomes dissolved in the fluoride layer 39.

As an example of operation according to the invention a trace of aluminium is maintained in the molten metal of the bath, and aluminium oxide formed by reaction of oxygen in the molten metal with the aluminium rather than with the molten metal rises to the surface of the pocket of molten metal. A layer of sodium aluminium fluoride (cryolite) with some dissolved aluminium oxide is confined on the surface of the pocket. An electrode 40, preferably a carbon electrode, is mounted in the headspace 37 on an electrically conductive support 31 which passes through the outer wall 35 of the pocket. The position of the electrode 40 is so adjusted that the bottom of the electrode dips into the layer of fluoride 39 but does not contact the molten metal in the pocket.

Electrical connection is made to the molten metal in the pocket by means of a second carbon bar 42 which is mounted horizontally in the side wall 35 near the floor 33 of the pocket. Terminals 43 and 44 are provided for connection to a source of direct current, which is so connected to the terminals 43 and 44 that the electrode 40 is a positive electrode and the metal in the pocket 31 is the negative electrode.

When current is passed between the electrode 40 and the molten metal in the pocket 31 electrolysis of the layer 39 takes place. The layer 39, or at least that part of it which contacts the molten metal, is in a molten state, which molten state is Iassured when the pocket 31 is positioned at such a distance from the outlet end of the bath that in the region where the pocket is located the temperature of the molten metal bath is high enough to keep the layer molten. Alternatively the pocket may be heated to keep the layer in a molten condition.

Other additive elements may be used in the bath, for example lithium, sod-ium, potassium, magnesium, calcium, strontium, barium, cerium, manganese, iron, zinc, boron, silicon, titanium, zirconium, niobium or tantalum. The layer 39 may be a layer of a simple fluoride, for example calcium fluoride when calcium is used as the additive element; or magnesium fluoride when magnesium is the additive element. Another compound, for example sodium fluoride, may be added to lower the melting point of the layer 39 or to facilitate electrolysis.

Two other preferred ways of operating according to the invention are with boron as the additive element and a layer of a mixture of potassium fluoborate (IQBFQ and potassium chloride, with dissolved boric oxide; or with magnesium as the additive element and a layer 39 of a mixture of magnesium fluoride, barium fluoride and sodium fluoride, with dissolved magnesium oxide. Lithium potassium fluoride may be employed.

When current is passed between the electrode 40 and the molten metal in the pocket 31, the molten metal in the pocket 31 becomes the negative electrode and electrolysis of the layer 39 takes place. Oxide dross taken up in the layer breaks down and the addi-tive element is released from the fluoride layer and returns to the molten metal in the pocket so that a fresh surface of the fluoride is presented to the molten metal at the interface and takes up fresh dross from the molten metal.

Diffusion of the additive element takes place throughout the bath of molten metal to maintain the desired trace of additive element in the bath. The overall concentration of additive element maintained in the bath in this way is, for example a concentration on the range of the order of to 50 parts per million.

As impurities are being removed continuously from the pocket or pockets there is a tendency for dross in the main part of the bath to migrate into the pocket or pockets where the dross is removed by solution in the layer 39. It has been found that by controlling the direct current supplied in known manner, for example a current of 100 amps at 5 volts, passing between the electrode 4t) and the molten metal in the pocket, impurities in the molten bath are maintained at a desired minimum.

As electrolysis proceeds the gas evolved, for example oxygen, with possibly some traces of chlorine or iluorine, is cleared from the headspace 37 by the flow of atmosphere maintained in the duct 38. Since the headspace 37 over the pocket is completely segregated from the headspace 14 over the main part of the bath there is no danger of impurities released in gaseous form into the headspace 37 returning to the -bath of molten metal.

In order to assist the removal of reaction products from the molten metal into the lfluoride layer and to carry freshly released additive metal into the bath, the bath metal may be circulated through the pockets 31. This embodiment of the invention is illustrated in FIGURES 3 to 5. Pockets 31 of molten metal of the bath are formed in one or both side walls 6 of the tank structure. Two pockets 31 are indicated in FIGURE 3 but there may be more than two pockets spaced at intervals down one or both side walls 6 of the tank structure. One of the pockets is illustrated in greater detail in FIGURES 4 and 5.

As shown in FIGURE 4 a pocket 31 of molten metal is formed in an extension 60 of the floor 5 of the tank structure. The pocket has a oor 61, an outer Wall 62 resting on the floor 61, and side walls 63. The walls 62 and 63 support a roof 64 extending between the top of the wall 62 and the side Wall 11a of the roof s-tructure which rests directly on the side wall 6 of the tank structure. There is thus a headspace 65 over pocket 31 of molten metal and a duct 38 connects with this headspace to enable a ow of atmosphere to be maintained through the headspace 65.

The pocket 31 communicates with the main part of the bath through channels, to be described below, which slope downwardly from the main part of the bath through the side wall 6 of the tank structure.

A layer 67 of a fluoride mel-t as described above is conned in the pocket, being supported on the surface of the molten metal in the pocket. The layer is contained by a carbon frame 68 which has a thin inner lining 69 of refractory material, preferably of a non-porous nature. The carbon frame 68 is impermeable to the fluoride melt, and the lining 69 acts as an electrically insulating layer to prevent current through the layer 67 taking an undersirable path through the carbon frame.

One electrode 70, preferably a carbon electrode, for example a graphite electrode, is mounted in the headspace 65 and passes up through the roof 64 of the pocke-t. The position of the electrode 70 is adjusted so that the bottom of the electrode dips into the fluoride layer 67 but does not contact the molten metal 31.

A connector bar 71 is shown in FIGURE 5, mounted in a side wall of one of the channels leading to the pocket and in contact with the molten metal. The electrode 70 and the bar 71 are connected to a source of direct current, which is so connected to the electrode 70 `and the bar 71 that the electrode 70 is a positive electrode and the pocket of molten metal 31, is a negative electrode.

When direct current is passed between the electrode 70 and the bar 71 via the iluoride melt and the molten metal in the pocket 31 electrolysis of the fluoride melt and the dissolved dross takes place, the layer 67 being in a molten state. This molten state is assured when the pocket 31 is located at positions in the bath where the temperature of the molten metal bath is high enough to keep the layer molten.

At -or near the outlet end of the bath however the temperature of the molten metal may not be suicient to maintain the layer molten. In such circumstances in order to keep layer 67 molten auxiliary heating electrodes 72 and 73 are mounted in the roof 64, being held in insulating bushings 74 which pass through the roof 64. The electrodes 72 and 73 dip into the layer 67 on either side of the main electrode 70 and are connected to a source of alternating current which passes between the electrodes through the compound and keeps the layer 67 molten. For example a current of 100 to 400 amperes may be passed, for example, and 5 kilowatts of power may be dissipated in heating the layer 67, which heating maintains the layer at a temperature of about 750 C. when the temperature of the molten metal in the pocket is about 620 C. This heating of the pocket enables the invention to be used right down to the cold end of the bath and extends the range of fluoride melts which may be used.

The auxiliary heating of the layer 67 enables a pocket 31 of molten metal to be positioned anywhere along the bath where scavenging of impurities from the molten metal is most desirable. It also serves to assist the electrolysis of the fluoride melt even when the temperature of the molten metal is sufficient to maintain the layer molten, because it has been found sometimes advantageous to maintain the temperature of the pocket of molten metal 31 higher than the general bath temperature in the region where the pocket is located. An upward flow of heated molten metal may also be induced in the pocket in the manner which will be described below thus giving an advantageous upward circulation of heated molten metal at the intersurface between the molten metal and the layer 67.

Preferably the molten metal is predominantly molten tin and the layer 67 is constituted in the same manner as the layer 39 described above.

The floor of the pocket is recessed as shown at 75 and an electric induction heater 76 is mounted in the oor under this recess. The inductor 76 has two legs 77 and 78 and causes an upward circulation of heated molten metal in the pocket. The molten metal is heated in the inductor so that there is a continual upward movement of heated metal towards the bottom of the layer 67 which movement assists in maintaining the layer in molten condition. Also the movement of molten metal towards the interface of the layer 67 and the molten metal assists the carrying of dross formed in the bath to the interface where it is taken up by the uoride melt, also the dilution of the additive element as it is released at the interface and the diffusion through the molten metal of the released additive element.

The pocket 31 communicates with the main part of the bath through two channels 79 and 80, which slope upwardly from the pocket to the bath. As shown in FIG- URE 5 the channels 79 and 80 are spaced apart longitudinally of the bath, the channel 79 being downstream of the channel 80, and the channel 80` being inclined in the upstream direction. The channel 79 is formed through the side wall of the tank structure, but the channel 80 is of L-shaped form extending through one of the side walls 63 of the pocket.

A paddle wheel impeller 81 is mounted in the channel 80, being mounted on the output shaft 82 of a gear box 83 which is driven by a motor 84 mounted outside the pocket. The paddle wheel 81 has peripheral vanes and impels the molten metal through the channel 80 in the direction of the arrow 85. This generates a circulation of molten metal through the pocket 31, metal being drawn into the pocket through the channel 79 as indicated by the arrow 86. By this flow of molten metal through the pocket oxide or sulphide dross is carried from the main part of the bath into the pocket and additive element released by the electrolysis in the pocket is immediately diluted and carried into the main part of the bath. The channels 79 and 80 are spaced sutliciently far apart down the side wall of the tank to avoid any substantial recirculation of molten metal from the channel 80 back into the channel 79. This is assisted by the sloping of the channel S which points away from the channel 79. If desired, the direction of ow of molten metal through the pocket may be reversed by reversing the motor 83.

By controlling in known manner the direct current, for example a current of 100 amps at 5 volts, passing between the electrode 70 and the bar 71 the removal of impurity from the molten metal is controlled.

Additive element released into the molten metal in the pocket is thus immediately diluted by the molten metal in the pocket and is then carried from the pocket into the main part of the bath where it reacts with impurities to form dross, which dross is brought into the pocket to be electrolysed after dissolution in the melt, and in this Way a state of equilibrum is setV up between the removal of dross into the molten compound in the pocket or pockets and the release of additive element by the electrolysis, so that a required overall concentration of additive element is maintained in the bath, for example a concentration in the range of the order of to 50 parts per million.

The invention thus provides in the manufacture of at glass in ribbon form during which the glass glides over a molten metal surface, means for causing a controlled removal of impurities from the molten metal while at the same time the level of trace element maintained in the molten metal is controlled. Dross formed by the scavenging action in the bath is taken up by the fluoride melt in the pocket, and further because of the regenerative nature of the process the maintenance of the desired concentration of additive element, and the bath and the molten layers which take up the dross only require attention from time to time.

The dross formed in the molten metal may be removed by a method according to the invention which does not involve electrolysis. A molten layer of liuoride, for example a layer of the kind described above is supported on the surface of each pocket of molten metal, and the layer is continuously decanted, being replaced by a fresh layer of the molten fluoride so that dissolved dr-oss is carried away in .the uoride decanted from the pocket and a fresh surface of the molten fluoride is continually presented to the surface of each pocket of molten metal.

Other compounds may be used in this decanting method, namely compounds which take up dross from the molten metal but which cannot be effectively electrolysed to separate the impurity from the additive element. For example layers of lithium potassium chloride, sodium potassium chloride or calcium barium chloride may be used. The molten layer which has taken up dross is decanted and fresh molten compound is -circulated over the surface of each pocket of molten metal so that the molten compound at the interface is continually renewed and reaction products which are taken up in the molten layer are carried away from the interface.

In all these examples of a non-electrolytic method of removing impurities from .the molten metal, the molten compound removed from the pockets is treated to remove the `dross carried in the molten compound, and may then be recirculated to recharge the molten layers supported on the pockets of molten metal.

The invention may be applied to any process for the manufacture of flat glass in ribbon form during which the glass is advanced over molten metal, for example a process in which a rolled ribbon of glass is delivered to a bath `of molten metal.

The invention also comprises at glass produced by a l@ method as described above and sheets of glass cut therefrom.

' We claim:

l1. In the manufacture of dat glass during which the glass is in contact with molten metal, continually maintaining in the molten metal a trace concentration of an additive element with which oxygen and sulphur present in the molten metal as impurity react in preference to reaction with the molten metal, and maintaining in contact with a Zone of the molten metal a molten layer at least partially comprising a member of the group consisting of fluorides and chlorides of said additive element, which molten layer takes up from the molten metal a product of reaction of said additive element with any said impurity in the molten metal.

2. In the manufacture of ilat glass during which the glass is in contact with a bath of a molten metal selected from the group consisting of molten tin and molten tin alloys in which tin predominates, continually maintaining in the bath a trace concentration of an additive element with which oxygen and sulphur present in the bath as impurity react in preference to reaction with the molten meal of the bath, and contining in contact with a zone of the bath a molten layer at least partially comprising a member of the group consisting of the uorides and the mixed chlorides of said additive element, which molten layer takes up from the bath of molten metal a product of reaction of said additive element with any said impurity in the molten metal.

3. A method according to claim 2, characterised by passing an electric current through the molten layer, and adjusting that current so as to cause electrolytic release of the impurity from the molten layer.

4. In the manufacture of at glass during which the glass is in contact with molten tin, continually maintaining in the molten tin a trace of an additive element selected from the group consisting of lithium, sodium, potassium,

magnesium, calcium, strontium, barium, cerium, manganese, iron, zinc, boron, aluminium, silicon, titanium, zirconium, niobium, and tantalum and maintaining in contact wth a zone of the molten tin a molten absorbing layer at least partially comprising a fluoride of said selected additive element.

5. A method according to claim 4, wherein the additive element is aluminium, and a confined layer of sodium aluminium fluoride (cryolite), with dissolved alumina, is supported on said zone.

6. A method according to claim 4, wherein the additive element is boron, and a confined layer of a mixture of potassium Huoborate and potassium chloride, with dissolved boric oxide, is supported on said zone.

7. A method according to claim 4, wherein the additive element is magnesium, and a conned layer of a mixture of magnesium fluoride, barium fluoride and sodium uoride, with dissolved magnesium oxide, is supported on said zone.

8. A method according to claim 3, wherein the molten layer is a layer of a mixed salt selected from the group consisting of lithium potassium chloride, sodium potassium chloride, calcium barium chloride and lithium potassium fiuoride.

References Cited UNITED STATES PATENTS 1,562,655 11/ 1925 Pacz 75-58 2,859,160 1l/1958 Helling 204-67 2,915,442 12/ 1959 Lewis 204-67 FOREIGN PATENTS 1,323,711 5/1963 France.

DONALL H. SYLVESTER, Primary Examiner. S. LEON BASHORE, Examiner. G. R. MYERS, Assistant Examiner. 

1. IN THE MANUFACTURE OF FLAT GLASS DURING WHICH THE GLASS IS IN CONTACT WITH MOLTEN METAL, CONTINUALLY MAINTAINING IN THE MOLTEN METAL A TRACE CONCENTRATION OF AN ADDITIVE ELEMENT WITH WHICH OXYGEN AND SULPHUR PRESENT IN THE MOLTEN METAL AS IMPURITY REACT IN PREFERENCE TO REACTION WITH THE MOLTEN METAL, AND MAINTAINING IN CONTACT WITH A ZONE OF THE MOLTEN METAL A MOLTEN LAYER AT LEAST PARTIALLY COMPRISING A MEMBER OF THE GROUP CONSISTING OF FLUORIDES AND CHLORIDES OF SAID ADDITIVE ELEMENT, WHICH MOLTEN LAYER TAKES UP FROM THE MOLTEN METAL A PRODUCT OF REACTION OF SAID ADDITIVE ELEMENT WITH ANY SAID IMPURITY IN THE MOLTEN METAL. 